1. Field of the Invention
The present invention relates to a design rule checking method, and more particularly to a design rule checking method applicable to a phase shift mask and a method of fabricating a phase shift mask.
2. Description of the Related Art
To produce a new semiconductor device, after the layout of the semiconductor device is designed, new masks used in lithographic processes are produced. Using these masks, the semiconductor device is fabricated. Pattern data for producing masks which includes units of pattern data for every layer of the fabrication process is produced as a final result of the layout design. Thereafter, design rule checking is performed so as to check whether or not the pattern data includes any errors, prior to the production of the masks. More specifically, in design rule checking, the pattern data for each layer is checked whether or not it satisfies design rules which define the minimum dimension or tolerance limit prescribed by a fabrication process technique, the degree of accuracy of registration of respective layers, and the like. If a new fabrication process technique is developed, new design rules may be required.
Recently, a lithographic technique using a phase shift mask has been proposed as a method for improving resolution in an optical lithographic technique. The term "resolution" refers to how small the line width and space width of a resist pattern can be made by a lithographic technique. As a method for improving resolution, a Levenson type phase shift mask is proposed (Levenson, M. D. et al., IEEE Transaction on Electron Devices, vol. ED-29, no. 12, pp. 1828-1836, December 1982).
A phase shift mask has a film, which is called a phase shifter, for shifting the phase of an exposure light on part of the mask. The degree of phase shift can be changed by adjusting the refractive index and thickness of the phase shifter. It is known that two beams of light having different phases can interfere with each other. For example, if two beams of light of equal intensity but which have phases shifted by 180 degrees from each other are superposed, the intensity of the composite light becomes zero.
The Levenson type phase shift mask utilizes the above principle. In other words, the phases of two beams of light which are transmitted through adjacent light transmitting portions are different by 180 degrees from each other. Therefore, the light is prevented from diffusing out into the shielding portion between the two light transmitting portions due to a diffraction effect. The light transmitting portion is a portion of the mask pattern produced based on pattern data through which light is transmitted. In the case where an edge of a phase shifter is positioned in the light transmitting portion, unwanted dark portions occur in the transmitted light due to interference. Therefore, the Levenson type phase shift mask is used for a negative resist in which it is not necessary to position the edge of the phase shifter in the light transmitting portion.
The phase shift mask of a single layer has a portion where two beams of light, which are transmitted through adjacent light transmitting portions, have the same phase, and a portion where two beams of light which are transmitted through adjacent light transmitting portions have different phases from each other. In the latter portion where two beams of light which are transmitted through the adjacent light transmitting portions have different phases from each other, a resist pattern with a high resolution can be formed as compared with the conventional resist pattern. On the other hand, in the other portion, the resolution is the same as in the prior art. Therefore, the minimum dimension realized by a lithographic technique is different from portion to portion of the phase shift mask of a single layer.
The software for performing a conventional design rule checking does not have a function capable of checking pattern data of such a phase shift mask. Accordingly, new software which is applicable to such a phase shift mask is required. However, in order to develop the new software, much time and money is required.